Molecular beam epitaxy (MBE) is a thin film deposition technique for depositing single-crystal semiconducting, insulating or metallic materials used in state-of-the-art electronic and opto-electronic devices. A material is grown by directing the output of effusion cells onto a heated substrate in an ultra high vacuum (UHV) chamber, at pressures as low as 10−10 Torr.
Most MBE applications require accurate control over the particle fluxes, and methods for continuous or periodic recalibration of the flux are necessary. Current generation MBE machines do not have accurate and immutable control of growth rates and composition of the deposited material because they rely on pre-growth calibrations of the beam flux using inaccurate and insensitive flux detectors that are themselves subject to calibration drift.
The most common method of flux monitoring in MBE involves the ionization of the particles in the beam flux and the detection of the number of those ions produced. For example, an ionization gauge can be used to measure the beam equivalent pressure (BEP) of each beam flux when run individually. However, ion gauges are typically not sensitive enough to measure doping fluxes accurately. Increased sensitivity and mass selectivity can be achieved by the use of, for example, a quadrupole mass filter followed by an avalanche electron multiplier. However these devices do not offer the required reproducibility in many cases. The primary problem for these types of detectors is that the calibration factor between the BEP and the actual particle flux in the beam changes over time because of detector sensitivity changes due to changes in the geometry of the sensor (the position of the filament with respect to the grid) or due to contamination and damage resulting from the exposure to background gasses or the particle beam being measured. For this reason, these devices require regular calibrations against actual measurements of thin film thickness and composition.
Another flux measurement device is the quartz crystal oscillator. Its operation relies on the measurement of the change in its oscillation frequency as it becomes loaded with particle deposit from the beam being measured. These oscillators provide a measure of the total thickness of the deposited material and the particle flux is determined by the time rate of change in this thickness. Quartz crystal oscillators have much lower sensitivity than ionization gauges, and they exhibit a variable sensitivity, a limited lifetime and reproducibility due to crystal overloading. Crystal overloading presents a significant limitation particularly for silicon deposition.
A third flux detection method involves electron-induced emission detection in which the particles in the flux are excited by bombardment with an electron beam and the subsequent photo-emission from the particles provides a measure of their number. As with quartz crystal oscillators, these detectors have a limited sensitivity and must be placed close to the beam source for adequate signal strength. In addition, they need to be calibrated against the actual thickness of deposited epilayers. Finally, they must be re-calibrated if the detector is moved or if the electron filament is replaced.
The beam flux can be also be inferred from measurements of the epilayer thickness using reflection high energy electron diffraction (RHEED). However, RHEED is not sensitive enough to measure doping fluxes, and it can result in kinetic damage to the surface and carbon contamination in certain materials. These limitations are described in more detail in Molecular Beam Epitaxy: Applications to Key Materials, edited by Robin F. C. Farrow, Noyes Publications, New Jersey, USA (1995); pages 76-86.
In addition to beam flux measurements in MBE applications, ionization based detectors (ionization gauges and residual gas analyzers) are commonly used to provide measurements of the vapor pressure of gases in ultra-high vacuum. Electrons are emitted from an emission source (such as a hot filament) and ionize particles which enter the detector volume. The resulting ions are collected on a grid and the residual gas density, or pressure, is inferred from the ratio of the ion current to the electron emission current.
These ion gauge type sensors have numerous disadvantages, as disclosed in multiple references (P. C. Arnold and S. C. Borichevsky, Nonstable behavior of widely used ionization gauges, J. Vac. Sci. Technol. A 12, 568 (1994); D. G. Bills, Causes of nonstability and nonreproducibility in widely used Bayard-Alpert ionization gauges, J. Vac. Sci. Technol. A 12, 574 (1994); P. C. Arnold, D. G. Bills, M. D. Borenstein, and S. C. Borichevsky, Simple and reproducible Bayard-Alpert ionization gauge, J. Vac. Sci. Technol. A 12, 580 (1994), C. R. Tilford, A. R. Filippelli, and P. J. Abbott, Comments on the stability of Bayard-Alpert ionization gages, J. Vac. Sci. Technol. A 13, 485 (1995), and K. Jousten, A. R. Filippelli, C. R. Tilford, and F. J. Redgrave, Comparison of the standards for high and ultrahigh vacuum at three national standards laboratories, J. Vac. Sci. Technol. A 15, 2395 (1997).). The primary limitation is that the particle ionization efficiency depends on the particle species and on the precise geometry of the electron emitter and ion collector. For this reason, these devices require calibration for accurate measurements and they suffer from calibration drift due to changes in the tube ionization efficiency (from electron filament or ion collector sag). These devices include hot electron emission filaments that produce a variety of contaminant gases during operation that change and in some cases degrade the vacuum it is intended to measure. The ultimate sensitivity and accuracy of such gauges is limited by outgassing of the components and leakage current not associated with the presence of ionized background gas, both of which can change with the age of the detector.
Accordingly, a need exists for an improved method and device for measuring, with accuracy and precision, the flux from effusion cells in MBE systems. It would be even more advantageous if said method and device could be used for measuring the ambient density of any gas in any high or ultra-high vacuum system, allowing it to be used directly as a pressure sensor or as a calibration standard for other pressure sensors.
While the loss of particles (atoms and molecules) from confining potentials (magnetic or optical) due to interactions (collisions) with un-trapped particles residing in the ultra-high vacuum environment is a known empirical phenomenon, the actual use of these losses to accurately determine the absolute flux of particles into the trap volume has never been done. This is, in part, due to a variety of reasons including the non-obvious dependence of the trap loss rate on the interaction potentials, on the internal state of the trapped particle, and on the depth of the confining potential.
The present invention mitigates and/or obviates the above-noted disadvantages. In particular, the present invention discloses an apparatus and method to achieve accurate beam flux or ambient gas density determination from measurements of the particle loss rate from a trap.